1. Faculty of Physics and Mathematics, University of Latvia Zel¸l¸u iela 8, LV-1002 Riga, Latvia; Institute of Mathematics and Computer Science of University of Latvia Rai¸na bulvaris 29, LV-1459 Riga, Latvia
2. Institute of Solid State Physics of University of Latvia K¸ engaraga iela 8, LV-1063 Riga, Latvia
3. Faculty of Physics and Mathematics, University of Latvia Zel¸l¸u iela 8, LV-1002 Riga, Latvia; Institute of Mathematics and Computer Science of University of Latvia Rai¸na bulv¯aris 29, LV-1459 Riga, Latvia
4. Faculty of Exact Sciences, University of Craiova A.I. Cuza Street 13, 200585 Craiova, Romania